Semiconductor device having high breakdown voltage and method of manufacturing the same

ABSTRACT

In a semiconductor device with a high breakdown voltage, insulating layers are buried at regions in n -   silicon substrate located between gate trenches which are arranged with a predetermined pitch. This structure increases a carrier density at a portion near an emitter, and improves characteristic of an IGBT of a gate trench type having a high breakdown voltage.

This application is a continuation of application Ser. No. 08/762,175filed Dec. 9, 1996 now U.S. Pat. No. 5,894,149.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having a highbreakdown voltage and a method of manufacturing the same. Moreparticularly, the present invention relates to a semiconductor devicehaving a high breakdown voltage for use in a high-voltage inverter orthe like, and a method of manufacturing the same.

2. Description of the Background Art

Semiconductor devices having a high breakdown voltage for use inhigh-voltage inverters or the like have recently been demanded to have ahigher operation speed and a lower on-voltage in order to improve anoperation efficiency and an operation controllability of thehigh-voltage inverters. In a field of a class of thousands of volts, GTO(Gate Turn-Off) thyristors have been largely used. However, it has beenrecently studied to improve breakdown voltages of IGBTs (Insulated GateBipolar Transistors) which allow increase in speed of devices.

Works are now being made to develop IGBTs of a gate trench type whichcan increase a supply capacity of electrons by microscopic processing.However, achievement of high operation speed and low on-voltageunpreferably causes reduction in a breakdown voltage, and therefore itis necessary to improve limits for them.

Referring to FIG. 49, description will be given on a structure of anIGBT of a gate trench type having a high breakdown voltage which hasbeen studied.

FIG. 49 is a schematic cross section of an IGBT of a gate trench typehaving a high breakdown voltage.

The gate trench type IGBT having a high breakdown voltage includes alightly doped n⁻ silicon substrate 1 and p-wells 4 which are formed ofp-type impurity diffusion regions formed at a first main surface (uppersurface in the figure) of n⁻ silicon substrate 1. Gate trenches 70extending from p-wells 4 into n⁻ silicon substrate 1 are arranged with acertain pitch. Each gate trench 70 is formed of a gate trench groove 7ahaving a depth similar to the above pitch, a gate insulating film 7arranged on an inner surface of gate trench groove 7a and a gateelectrode 8 arranged inside gate insulating film 7.

At portions of p-wells 4 contiguous to first main surfaces of gatetrenches 70, there are arranged n⁺ emitter regions 5 formed of heavilydoped n-type impurity diffusion regions.

Portions of gate electrode 8 and gate insulating film 7 of each gatetrench 70 which are protruded beyond the first main surface are coveredwith a silicate glass film 19. There is also formed an emitter electrode10, which covers entirely the first main surface, is formed of, e.g., ametal film, and is electrically connected to n⁺ emitter regions 5 andp-wells 4.

An n-buffer layer 2 formed of an n⁺ impurity diffusion region isarranged at a second main surface (lower surface in the figure) of n⁻silicon substrate 1. A p-collector region 3 made of a p⁺ type impuritydiffusion region is formed at a surface of n-buffer layer 2. A collectorelectrode 11 made of, e.g., a metal film is arranged at a surface ofp-collector region 3. n-buffer layer 2 which is designed as a so-calledpunch through type is employed for improving a precision of thesemiconductor device, and is not essential.

Operations of the above gate trench type IGBT having a high breakdownvoltage will be described below.

First, an operation in an off state will be described below. A voltageis applied across collector electrode 11 and emitter electrode 10 whileapplying a voltage sufficiently lower than a gate threshold voltageacross gate electrode 8 and emitter electrode 10. Thereby, a junctionbetween n⁻ silicon substrate 1 and p-well 4 attains a reversely biasedstate, and a depletion layer extends mainly toward n⁻ siliconsubstrate 1. Since the gate potential is low, holes in p-well 4 areattracted to and accumulated at a surfaces of p-well 4 contiguous togate trench 70, so that the gate trench channel attains an off state.

An operation in an on state will be described below. A voltage isapplied across collector electrode 11 and emitter electrode 10 whileapplying a voltage sufficiently higher than the gate threshold voltageacross gate electrode 7 and emitter electrode 10. Thereby, a surfacecontiguous to gate trench 70 attracts electrons in p-well 4, because thegate potential is high. Therefore, n-inversion occurs, and a trenchchannel is formed. Thereby, electrons in n⁻ silicon substrate 1 aresupplied from n⁺ emitter region 5 into n⁻ silicon substrate 1 throughthe trench channel, and electrons flow toward p-collector layer 3carrying a positive potential.

When electrons flow into p-collector layer 3, holes are supplied fromp-collector layer 3 into n-buffer layer 2. These holes causeconductivity modulation in n⁻ silicon substrate 1. If a life time in n⁻silicon substrate 1 is sufficiently long, these holes reach the vicinityof the trench channel, and will be attracted into p-well 4 at a lowerpotential.

Description will now be given on a so-called turn-off state in which thestate changes from on state to the off state described above. In aninverter circuit which is typical application of a switching elementhaving a high breakdown voltage, an inductive load is controlled in manycases. FIG. 50 shows results of evaluation of the turn-off operation ina case where the inductive load is controlled in the conventionalhigh-breakdown-voltage IGBT of the gate trench type.

When charges accumulated in the gate capacity decrease and the gatevoltage lowers, a sufficient load current may not flow in thehigh-breakdown-voltage IGBT of the gate trench type, in which case acollector voltage rises. When the collector voltage exceeds 3000 V whichis a bus voltage of the inverter circuit, the load current bypasses theIGBT and flows through a bus circuit, so that the collector current inthe high-breakdown-voltage IGBT of the gate trench type decreases. Whenexcessive carriers, which were accumulated in n⁻ silicon substrate 1 andn-buffer layer 2 in the high-breakdown-voltage IGBT of the gate trenchtype during the on state, are discharged or released, the collectorcurrent of the high-breakdown-voltage IGBT of the gate trench type flowsno longer, and the turn-off operation is completed.

The high-breakdown-voltage IGBT of the gate trench type described abovesuffers from the following problem in the off state. A current otherthan a slight leakage current generated inside a depletion layer doesnot flow between collector electrode 11 and emitter electrode 10, and ahigh impedance is exhibited.

With increase in collector voltage, the depletion layer further extendsto n-buffer layer 2. The electric field in the IGBT increases as thevoltage rises. Although the potential at the bottom of gate trench 70 issubstantially equal to that at gate electrode 8, the potential, which n⁻silicon substrate 1 under p-well 4 carries at a position at the samedepth as the bottom, rises above the potential at p-well 4 (emitterpotential) due to donor ions between the above-mentioned position top-well 4. In particular, the electric field at a bottom corner in gatetrench 70 tends to increase.

In the above state, when the electric field inside the IGBT exceeds athreshold electric field and thereby tends to cause strong impact, aleakage current between collector electrode 11 and emitter electrode 10rapidly increases, resulting in breakdown of the IGBT.

In order to achieve a high breakdown voltage of the IGBT, therefore, itis necessary to increase a drop voltage which exists in the depletionlayer until the electric field reaches the threshold electric field. Forthis purpose, the thickness of n⁻ silicon substrate 1 is increased so asto lower an impurity concentration. Also, in order weaken the electricfield at the lower corner of gate trench 70 and thereby increase thethreshold electric field, such a structure has been employed that thegate trench 70 has a round lower corner, or that a distance between gatetrenches 70 is reduced (see the following reference 1).

Reference 1: K Matsushita, I Omura and T Ogura, "Blocking Voltage DesignConsideration for Deep Trench MOS Gate High Power Devices" Proc., ISPSD'95, pp. 256-260.

However, reduction in a distance between gate trenches 70 increases anarea of gate trenches 70 per unit area, which unpreferably increases thegate capacity and provides a severe limit to processing for fabricatingthe IGBT.

Then, a problem during the on state will be described below.

A density of electrons and holes in n⁻ silicon substrate 1 increases anda low impedance is attained between collector electrode 11 and emitterelectrode 10. However, a relatively large number of holes are attractedinto p-well 4. This restricts introduction of electrons from the trenchchannel into n⁻ silicon substrate 1.

A conventional IGBT, which has been studied for a practical use,exhibits such a carrier density distribution that a carrier density nearthe collector electrode is higher than that near the emitter electrodeas shown in FIG. 51.

The on-voltage can be lowered by strengthening the conductivitymodulation of n⁻ silicon substrate 1. The on-voltage lowers inaccordance with increase in lifetime of carriers in n⁻ silicon substrate1, increase in supply of electrons from the trench channel side andincrease in supply of holes from p-collector layer 3. Particularly inthe IGBT of a class of thousands of volts, supply of an excessivelylarge amount of holes from p-collector layer 3 causes a problem, so thatsuch a design is required that electrons can be supplied from the trenchchannel side as much as possible.

In order to increase the supply of electrons from the trench channelside, it is necessary to reduce an amount of holes flowing into p-well4. For achieving this, the following structures have been proposed inthe prior art:

(i) Structure in which a pitch of gate trenches is reduced (seereference 2).

(ii) Structure in which gate trenches have a large depth (see reference2).

(iii) Structure which corresponds to the structure of IGBT shown in FIG.49 and includes a heavily doped n-type layer under p-well 4.

(iv) Structure in which an emitter contact of p-well 4 and a portion ofgate trench 70 not provided with n-emitter region 5 are inserted betweentrench IGBT portions (see FIG. 50, and references 2 and 3).

Reference 2: M Kitagawa, A Nakagawa, K Matsushita, S Hasegawa, Y Inoue,A Yahata and H Takenaka "4500V IEGTs having Switching CharacteristicsSuperior to GTO" Proc. ISPSD' 95, pp. 485-491.

Reference 3: Japanese Patent Laying-Open No. 7-50405 (1995).

However, if IGBTs were designed as described above, the structures of(i), (ii) and (iv) would suffer from a problem that increase in gatecapacity, and the structures of (ii) and (iii) would suffer from aproblem of lowering of breakdown voltages. The former problem isgeometrically apparent from the fact that an area ratio of the gateinsulating film is large. An example of the latter problem will bedescribed below with reference to FIG. 53, which shows results ofevaluation of breakdown voltages and saturation voltages in IGBTs of aclass of 4500 volts with various heavily doped n-type layers formed atvarious depths under p-wells 4 and having different impurityconcentrations. As structure parameters of the reference IGBT for theabove evaluation, the impurity concentration of n⁻ silicon substrate 1is 1.3 e13/cm³, a thickness is 625 μm, a pitch of gate trenches 70 is 5μm and a depth thereof is 5 μm.

As shown in FIG. 53, the saturation voltage is certainly lower than thatof the reference IGBT (represented as reference TIGBT in the table).However, as the saturation voltage decreases to a higher extent, thebreakdown voltage also decreases to a higher extent, so that it isimpossible to find practically acceptable conditions of the impurityconcentration and the position of the n-type layer.

A problem caused by the turn-off operation will be described below.

Referring again to FIG. 50, there is a range indicated by Z in thefigure, in which the rapidly raised collector voltage (V_(CE)) of about1200 V rises slowly to about 3000 V. With reference to a cumulativewaveform of a switching loss (E_(OFF)) represented by broken line, amajor portion of turn-on loss is consumed at the range indicated by Z.

As characteristics of IGBT shown in FIG. 50, there are shown waveformsof an element of which saturation voltage is set to about 3 V bycontrolling introduction of holes from p-collector layer 3. Thestructure parameters are as follows. The impurity concentration ofsilicon substrate is 1.0 e13/cm³, a thickness is 425 μm, a gate trenchpitch is 5.3 μm, a depth is 5 μm and a width is 1 μm.

The above phenomenon has been elucidated as follows by analyzing aninternal state of the IGBT used for the device simulation. When carriersaccumulated in the IGBT are discharged and the collector voltage rises,the depletion layer does not extend rapidly from the emitter electrodeside if a large amount of carriers are accumulated at a neutral regionof n⁻ silicon substrate 1 near the collector electrode, so that thecollector voltage rises slowly.

At the same time, the following phenomenon occurs. A difference incharge density between holes and electrons forming a current acts tomodulate and therefore enhance the electric field in the depletionlayer, and impact-generated carriers transitionally supply an electroncurrent, which delays turn-off.

In order to suppress the above phenomenon for reducing the turn-offloss, it is necessary to employ a design which can avoid excessiveaccumulation of carriers at the neutral region in n⁻ silicon substrate 1near the collector electrode in the on state. However, mere suppressionof introduction of holes from p-collector layer 3 would result in riseof the saturation voltage and therefore increase in the on-state loss.

SUMMARY OF THE INVENTION

An object of the invention is provide a semiconductor device having ahigh breakdown voltage, which can achieve a high breakdown voltagewithout increasing a gate capacity in an off state of ahigh-breakdown-voltage IGBT of a gate trench type, and a method ofmanufacturing the same.

Another object of the invention is provide a semiconductor device havinga high breakdown voltage, which can reduce a saturation voltage withoutreducing a breakdown voltage in an on state of a high-breakdown-voltageIGBT of a gate trench type, and a method of manufacturing the same.

A still another object of the invention is provide a semiconductordevice having a high breakdown voltage, which can reduce a turn-off lossin a turn-off operation of a high-breakdown-voltage IGBT of a gatetrench type, and a method of manufacturing the same.

According to an aspect of the invention, a semiconductor device with ahigh breakdown voltage includes a semiconductor substrate of a firstconductivity type having first and second main surfaces; a firstimpurity layer of a second conductivity type formed at the first mainsurface; a gate trench formed of a groove extending from the firstimpurity layer into the semiconductor substrate, a gate insulating filmcovering an inner surface of the groove, and a gate electrode fillingthe groove and formed of an electric conductor; a pair of impurityregions of the first conductivity type formed near a surface of thefirst impurity layer and located at opposite sides of the gate trench; afirst main electrode layer formed over the first main surface, opposedto the gate trench with an insulating film therebetween and electricallyconnected to the impurity region and the first impurity layer; a secondimpurity layer of the second conductivity type formed at the second mainsurface; and a second main electrode layer formed at a surface of thesecond impurity layer. The gate trench is formed at each of positionsspaced with a predetermined pitch. An insulating layer is arranged at aposition in the semiconductor substrate located between the gatetrenches.

Further, according to a method of an aspect of the invention and morespecifically a method of manufacturing a semiconductor device with ahigh breakdown voltage, a step is performed for preparing a firstsemiconductor substrate of a first conductivity type provided at itsmain surface with an insulating layer. Thereafter, a secondsemiconductor substrate of the first conductivity type is arranged overthe insulating layer to form a semiconductor substrate having first andsecond main surfaces and including the insulating layer interposedtherebetween.

Then, a first impurity layer of the second conductivity type is formedat the first main surface of the semiconductor substrate. Thereafter, animpurity region of the first conductivity type is formed at apredetermined region of a surface of the first impurity layer.

Then, a second impurity layer of the second conductivity type is formedat the second main surface. Thereafter, a groove extending to theinsulating layer is formed at the impurity region.

Then, the insulating layer exposed at the groove is removed. Thereafter,an epitaxial growth layer having the same impurity concentration as thesemiconductor substrate is formed at an inner surface of the groove byan epitaxial growth method.

Then, a gate insulating film is formed at a surface of the epitaxialgrowth layer in the groove. Thereafter, the groove is filled with anelectric conductor to form a gate electrode.

Then, a portion of the gate electrode exposed at the first main surfaceis covered with an insulating film. Thereafter, a step is performed toform a first main electrode layer covering the first main surface andelectrically connected to the first impurity layer and the impurityregion. Thereafter, a second main electrode layer is formed at thesecond main surface.

In a method of manufacturing a semiconductor device with a highbreakdown voltage according to another aspect of the invention, a stepis performed to form a first semiconductor substrate of a firstconductivity type provided at its main surface with insulating layerswith a predetermined pitch. Thereafter, a step is performed to form asecond semiconductor substrate of the first conductivity type providedat its main surface with concavities of the same width and thickness asthe insulating layer with the same pitch as the insulating layers at themain surface.

Then, the main surfaces of the first and second semiconductor substratesare laminated together to form a semiconductor substrate having firstand second main surfaces and including the insulating layers interposedtherebetween with a predetermined pitch. Thereafter, a first impuritylayer of a second conductivity type is formed at the first main surfaceof the semiconductor substrate.

Then, an impurity region of the first conductivity type is formed at apredetermined region of the surface of the first impurity layer.Thereafter, a second impurity layer of the second conductivity type isformed at the second main surface.

Then, a groove extending to the semiconductor substrate through a regionbetween the insulating layers are formed at the impurity region.Thereafter, a gate insulating film is formed at an inner surface of thegroove.

Then, the groove is filled with an electric conductor to form a gateelectrode. Thereafter, a portion of the gate electrode exposed at thefirst main surface is covered with an insulating film.

A step is performed to form a first main electrode layer electricallyconnected to the first impurity layer and the impurity region andcovering the first main surface. Thereafter, a second main electrodelayer is formed at the second main surface.

According to the semiconductor device with a high breakdown voltage andthe method of manufacturing the same described above, the insulatinglayer is arranged at the position in the semiconductor substrate betweenthe gate trenches.

This insulating layer functions as a kind of capacitor during the offstate of the semiconductor device having a high breakdown voltage.Electrons are attracted to the upper surface of the insulating layer toform strongly negative space charges. These strongly negative spacecharges intercept an electric field which would be gradually enhanced bydonor ions and would be applied from the lower side of the semiconductorsubstrate toward the first impurity layer, so that an electric field isnot substantially present between the insulating layer and the firstimpurity layer. Thereby, the potential on the upper surface of theinsulating layer lowers to a potential substantially equal to that onthe first main electrode connected to the first impurity layer.

Meanwhile, the interior of the gate trench carries a potential nothigher than that on the first main electrode, and an electric field isenhanced at a corner of the bottom of the gate trench. However, if anend of the insulating layer is close to the corner of the bottom of thegate trench, a potential difference between them is small and theelectric field is weakened, because the potential under the insulatinglayer is low. Consequently, the breakdown voltage can be improved.

During the on state of the semiconductor device having a high breakdownvoltage, the insulating layer functions to prevent attraction of holesinto the first impurity layer. Since the gate trench is strongly andpositively biased, electrons are attracted to a wall of the gate trench,and holes are repelled, so that holes cannot easily pass between thewall of gate trench and the insulating layer, and cannot substantiallyarrive at the first impurity layer. Therefore, the hole currentdecreases and an efficiency of introduction of electrons from the trenchchannel increases, so that a great amount of electrons and holes aresupplied to the semiconductor substrate, increasing the conductance.Therefore, the saturation voltage can be reduced.

In addition, when this semiconductor device having a high breakdownvoltage is employed as an IGBT, increase in carrier density within thesemiconductor substrate of the first conductivity type at on state wouldlead to increase in conductivity of this semiconductor substrate anddecrease in saturation voltage. At this time, however, if the holesupply from the impurity layer of the second conductivity type isreduced to recover the saturation voltage, the exhibited distribution ofthe carrier density would be such that it is higher at the side of thefirst main electrode than at the side of the second main electrode.

When the semiconductor device with a high-breakdown-voltage is turnedoff, the gate voltage lowers, and the channel cannot supply a sufficientamount of electrons, so that the voltage on the second main electrodelayer starts to rise. When this rise starts, excessive holes which havebeen accumulated in the semiconductor substrate are attracted toward thegate trench at a low voltage, move along the wall of the gate trench tothe first impurity layer, and then flow into the first main electrodelayer.

Since a large current would not flow through a portion located under thefirst impurity layer and surrounded by the gate trenches even ifinsulating layer were not present at this portion, the insulating layerexisting at this portion does not cause a particular problem. During theon state, if an element having a carrier distribution indicated by solidline is turned off, holes which are relatively large in amount near thefirst main electrode layer are discharged from the first main electrodeside, so that the depletion layer formed by discharge of holes extendsonly slowly at an initial stage in the turn-off operation, and thesecond main electrode voltage starts to rise relatively slowly.

However, when the second electrode voltage rises and the depletion layerextends to some extent, the end of the depletion layer is located at aregion in which only a small amount of initially accumulated carriersare present, and the depletion layer extends rapidly owing to dischargeor release of holes. Thereby, the collector voltage rapidly rises untilthe end of the turn-off operation. Consequently, the turn-off loss isreduced, and thereby it is possible to suppress increase in thetemperature inside the semiconductor device with a high breakdownvoltage.

According to still another aspect of the invention, a semiconductordevice with a high breakdown voltage includes a semiconductor substrateof a first conductivity type having first and second main surfaces; afirst impurity layer of a second conductivity type formed at the firstmain surface; a gate trench having a first groove extending from thefirst impurity layer into the semiconductor substrate, a gate insulatingfilm covering an inner surface of the first groove, and a gate electrodefilling the first groove and made of an electric conductor; a pair ofimpurity regions of the first conductivity type formed near a surface ofthe first impurity layer and located at opposite sides of the gatetrench; a first main electrode layer covering the first main surface,opposed to the gate trench with an insulating film therebetween andelectrically connected to the impurity region and the first impuritylayer; a second impurity layer of the second conductivity type formed atthe second main surface; and a second main electrode layer formed at asurface of the second impurity layer. The gate trench is formed at eachof positions spaced from each other with a predetermined pitch. At aposition in the semiconductor substrate located between the gatetrenches, there is arranged an emitter trench having a second grooveextending from the first impurity layer into the semiconductorsubstrate, an insulating film covering an inner surface of the secondgroove, and a second electrode filling the second groove andelectrically connected to the first main electrode layer.

According to yet another aspect of the invention, a method ofmanufacturing a semiconductor device with a high breakdown voltageincludes a step of preparing a semiconductor substrate of a firstconductivity type having first and second main surfaces. Thereafter, afirst impurity layer of a second conductivity type is formed at thefirst main surface of the semiconductor substrate.

A plurality of impurity regions of the first conductivity type areformed at predetermined regions in a surface of the first impuritylayer, respectively. Thereafter, a second impurity layer of the secondconductivity type is formed at the second main surface.

Then, a first groove extending to the semiconductor substrate is formedat the impurity region. Thereafter, a second groove extending to thesemiconductor substrate is formed at a portion of the first impuritylayer.

First insulating films are formed at inner surfaces of the first andsecond grooves. Thereafter, the first and second grooves are filled withelectric conductors to form a buried gate electrode and a buried emitterelectrode.

Portions of the buried gate electrode and the buried emitter electrodeexposed at the first main surface are covered with a second insulatingfilm. Thereafter, a contact hole extending to the buried emitterelectrode is formed at the second insulating film formed on the buriedemitter electrode.

Then, a step is performed to form a first main electrode layer coveringthe first main surface and electrically connected to the first impuritylayer, the impurity region and the buried emitter electrode. Thereafter,a second main electrode layer is formed at the second main surface.

According to the semiconductor device with a high breakdown voltage andthe method of manufacturing the same described above, the emitter trenchset to the same potential as the first main electrode is arrangedbetween the gate trenches.

The above structure can further reduce the saturation voltage, and canincrease the amount of carriers introduced into the semiconductorsubstrate. Also, the structure can slightly increase the breakdownvoltage. Therefore, a performance of the semiconductor device with ahigh breakdown voltage can be improved.

According to this structure, since the emitter trench is set to apotential equal to that of the first main electrode, a unit area of thegate trenches is small, so that the gate capacity can be significantlyreduced. In particular, reduction in the capacity (feedback capacity)between the gate trench and the second main electrode layer allows rapidswitching, and therefore can achieve an effect of reducing a switchingloss. This is strongly demanded in the semiconductor device with a highbreakdown voltage, of which purpose is to handle a large power, andreduction in the gate capacity is strongly demanded for simplifyingdriving circuit and improving response time. Therefore, the above pointis very important.

According to further another aspect of the invention, a semiconductordevice with a high breakdown voltage includes a semiconductor substrateof a first conductivity type having first and second main surfaces; afirst impurity layer of a second conductivity type formed at apredetermined region in the first main surface; a gate trench having afirst groove formed at the region provided with the first impurity layerand extending from the first impurity layer to the semiconductorsubstrate, a gate insulating film covering an inner surface of the firstgroove and an electrode filling the first groove and made of an electricconductor; a pair of impurity regions of the first conductivity typeformed near a surface of the first impurity layer with the gate trenchtherebetween; a first main electrode layer opposed to the gate trenchwith an insulating film therebetween, electrically connected to theimpurity region and the first impurity layer and covering the first mainsurface; a second impurity layer of the second conductivity type formedat the second main surface; and a second electrode layer formed at thesecond impurity layer. The gate trench is formed at each of positionsspaced from each other with a predetermined pitch. At positions betweenthe gate trenches and spaced from each other with a predetermined pitch,there are arranged a plurality of emitter trenches each having a secondgroove extending from the first impurity layer into the semiconductorsubstrate, an insulating film covering an inner surface of the secondgroove, and a second electrode filling the second groove andelectrically connected to the first main electrode layer.

According to a further aspect of the invention, a method ofmanufacturing a semiconductor device with a high breakdown voltageincludes a step of preparing a semiconductor substrate of a firstconductivity type having first and second main surfaces.

Then, an impurity region of the first conductivity type is formed at apredetermined region of the first main surface. Thereafter, a secondimpurity layer of the second conductivity type is formed at the secondmain surface. Then, a first groove extending to the semiconductorsubstrate is formed at a predetermined position in the impurity region.Thereafter, a plurality of second grooves are formed at thesemiconductor substrate defined by the impurity region.

First insulating films are formed at inner surfaces of the first andsecond grooves. Thereafter, the first and second grooves are filled withelectric conductors to form a buried gate electrode and a buried emitterelectrode, respectively.

Portions of the buried gate electrode and the buried emitter electrodeexposed at the first main surface are covered with a second insulatingfilm. Thereafter, a contact hole extending to the buried emitterelectrode is formed at the second insulating film formed on the buriedemitter electrode.

Then, a step is performed to form a first main electrode layer coveringthe first main surface and electrically connected to the semiconductorsubstrate, the impurity region and the buried emitter electrode.Thereafter, a second main electrode layer is formed at the second mainsurface.

According to the semiconductor device with a high breakdown voltage andthe method of manufacturing the same described above, a plurality ofemitter trenches which are set to the same potential as the first mainelectrode layer are disposed between the gate trenches. Owing to thisstructure, it is possible to reduce a ratio of a distance between thegate trench and the emitter trench with respect to the pitch of the gatetrenches to a necessary value, even if the gate trench and the emittertrench have the same configuration. Therefore, the semiconductor devicecan be manufactured easily.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section showing a structure of ahigh-breakdown-voltage IGBT of a gate trench type according to anembodiment 1 of the invention;

FIG. 2 shows dependency of a breakdown voltage and a saturation voltageon vertical specifications of an insulating layer 1 in the embodiment 1of the invention;

FIG. 3 shows dependency of a breakdown voltage and a saturation voltageon lateral specifications of an insulating layer 1 in the embodiment 1of the invention;

FIG. 4 shows dependency of a breakdown voltage and a saturation voltageon specifications of a gate trench pitch in the embodiment 1 of theinvention and the conventional IGBT;

FIG. 5 shows dependency of a breakdown voltage and a saturation voltageon a gate trench pitch and lateral specifications of an insulating layerin the embodiment 1 of the invention;

FIG. 6 shows saturation voltage characteristics of the structure of theembodiment 1 of the invention and the conventional structure forcomparison;

FIG. 7 shows inductive load turn-off characteristics in the embodiment 1of the invention;

FIG. 8 shows a vertical distribution of electron density in theembodiment 1 of the invention;

FIGS. 9 to 20 are cross sections showing 1st to 12th steps formanufacturing the high-breakdown-voltage IGBT of the gate trench type ofthe embodiment 1 of the invention, respectively;

FIGS. 21(a) to 29 are cross sections showing 1st to 9th steps formanufacturing a high-breakdown-voltage IGBT of a gate trench type of asecond form the embodiment 1 of the invention, respectively;

FIG. 30 is a cross section showing a structure of thehigh-breakdown-voltage IGBT of the gate trench type of an embodiment 2of the invention;

FIG. 31 shows dependency of a breakdown voltage and a saturation voltageon gate trench pitch specifications of the IGBT of the embodiment 2 ofthe invention and the conventional IGBT;

FIGS. 32 to 40 are cross sections showing 1st to 9th steps formanufacturing the high-breakdown-voltage IGBT of a gate trench typeaccording to the embodiment 2 of the invention, respectively;

FIG. 41 is a top view showing the pattern corresponding to FIG. 39,according to the embodiment 2 of the invention;

FIG. 42 is a cross section showing a high-breakdown-voltage IGBT of agate trench type according to an embodiment 3 of the invention;

FIGS. 43 to 46 are cross sections showing 1st to 4th steps formanufacturing the high-breakdown-voltage IGBT of the gate trench type ofthe embodiment 3 of the invention, respectively;

FIG. 47 is another first cross section showing a high-breakdown-voltageIGBT of a gate trench type according to embodiment 3 of the invention;

FIG. 48 is another second cross section showing a high-breakdown-voltageIGBT of a gate trench type according to embodiment 3 of the invention.

FIG. 49 is a cross section showing a high-breakdown-voltage IGBT of agate trench type in the prior art;

FIG. 50 shows inductive load turn-off characteristic of an IGBT in theprior art;

FIG. 51 shows vertical distribution of electron density of the IGBT inthe prior art;

FIG. 52 is a cross section showing an application structure of thehigh-breakdown-voltage IGBT of the gate trench type in the prior art;and

FIG. 53 shows dependency of a breakdown voltage and a saturation voltageon specifications of an n-layer buried under a p-well in thehigh-breakdown-voltage IGBT of the gate trench type in the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(Embodiment 1)

Description will now be given on a semiconductor device with a highbreakdown voltage and a method of manufacturing the same according to anembodiment 1 of the invention.

Referring to FIG. 1, a sectional structure of an IGBT of a gate trenchtype with a high breakdown voltage will be discussed below as an exampleof the semiconductor device with a high breakdown voltage of theembodiment 1. The high-breakdown-voltage IGBT of the gate trench type(will also be referred to merely as an "IGBT" hereinafter) includes alightly doped n⁻ silicon substrate 1. p-well 4 made of a p-type impuritydiffusion region is formed at a first main surface (upper surface inFIG. 1) of the silicon substrate 1.

Gate trenches 70 are formed at n⁻ silicon substrate 1 with apredetermined pitch. Each gate trench 70 is formed of a gate trenchgroove 7a, which is slightly deeper than p-well 4 and extends throughthe first main surface to a position at a depth similar to theabove-mentioned pitch, a gate insulating film 7 arranged on an innersurface of gate trench groove 7a and made of an oxide film, and a gateelectrode 8 arranged inside gate insulating film 7.

An n⁺ emitter region 5 made of a heavily doped n-type impurity diffusionregion is formed at a surface of p-well 4 contiguous to the first mainsurface of each gate trench 70. Portions of gate electrode 8 andinsulating film 7 exposed at the first main surface are covered with aninterlayer insulating film 19 made of, e.g., an oxide film. There isalso arranged an emitter electrode 10, which is electrically connectedto emitter regions 5 and p-wells 4, covers the first main surface and ismade of, e.g., a metal film.

An n-buffer layer 2 made of an n-type impurity diffusion region isarranged at a second main surface (lower surface in the figure) ofsilicon substrate 1, and a p-collector layer 3 made of a p-type impuritydiffusion region is arranged at the surface of n-buffer layer 2. Acollector electrode 11 made of, e.g., a metal film is formed at thesurface of p-collector layer 3. The purpose of n-buffer layer 2 is toimprove a performance in a design of a so-called punch through type, andis not essential.

As a distinctive feature in the structure of IGBT according to theembodiment 1, an insulating layer 15 made of a silicon oxide film isarranged at each of regions of n⁻ silicon substrate 1 located betweengate trenches 70.

Structural parameters of the IGBT shown in FIG. 1 are as follows. Theimpurity concentration of n⁻ silicon substrate 1 is 1.0 e13/cm³, and athickness (D) of n⁻ silicon substrate is 425 μm. The pitch of gatetrenches is 5.3 μm. A depth (d) and a width (W) of gate trench 70 are 5μm and 1 μm, respectively.

A thickness (Y') and a position (dx) of insulating layer 15 areimportant factors determining characteristics of the IGBT.

FIG. 2 shows results of evaluation of variation in a breakdown voltageand a saturation voltage corresponding to variation in longitudinalposition (Y) of insulating layer 15. A reference TIGBT in FIG. 2represents the results of IGBT shown in FIG. 47 and therefore notprovided with insulating layer 15.

It can be seen from FIG. 2 that provision of insulating layer 15 reducesthe saturation voltage, but the vertical position (Y) of insulatinglayer 15 is preferably at a level higher than the bottom of gate trench70 for improving the saturation voltage.

Meanwhile, insulating layer 15 having a thickness (Y') substantiallylower than 0.3 μm can improve the breakdown voltage as compared with theconventional structure of IGBT.

With respect to the thickness (Y') of insulating layer 15, arelationship of tradeoff is found between the breakdown voltage and thesaturation voltage to a certain extent, but a thinner insulating layer15 is preferable because the breakdown voltage takes priority over theother in the IGBT.

In the date shown in FIG. 2, since gate insulating film 7 in gate trench70 has a thickness of 0.075 μm, an appropriate relationship in thicknessbetween gate insulating film 7 and insulating layer 15 is that thethickness of insulating layer 15 is substantially smaller than aquadruple of that of gate insulating film 7. In this example, a distance(dx) from a wall surface of gate trench 70 to insulating layer 15 is 0.2μm.

FIG. 3 shows results of evaluation of variation in the breakdown voltageand saturation voltage corresponding to variation in lateral position(X) of insulating layer 15. It can be understood from data shown in FIG.3 that a shorter distance (dx) from the wall surface of gate trench 70to insulating layer 15 enhances an effect of reducing the saturationvoltage, and the breakdown voltage which exhibit a slight variationtakes on the maximum value with dx of about 0.2 μm.

For the structure of the conventional IGBT shown in FIG. 47 and thestructure of the IGBT of the embodiment 1 shown in FIG. 1, variation inthe breakdown voltage and saturation voltage was evaluated with variousvalues of pitch of gate trenches 70. The results are shown in FIG. 4. InFIG. 4, 2×TIGBT indicates a result obtained when the pitch of trenchgates 70 was doubled in the conventional IGBT shown in FIG. 47, and2×B.O.TIGBT indicates a result obtained when the pitch was doubled inthe IGBT shown in FIG. 1.

It can be seen from FIG. 4 that, as the pitch of the gate trenches 70increases in the IGBT of the conventional structure, the saturationvoltage increases and the breakdown voltage decreases, resulting inreduction in the performance of the IGBT. In the structure of IGBT ofthe embodiment, increase in the pitch can improve the breakdown voltage,although the improvement is achieved only to a slight extent. As itincreases to about 10 times (53 μm in this embodiment), the saturationvoltage decreases, and then the saturation voltage increases as itincreases to 20 times.

It is already found that, if the interface recombination speed betweeninsulating layer 15 and n⁻ silicon substrate 1 is large, the pitch whichminimizes the saturation voltage is small, and the effect of reducingthe saturation voltage is small.

FIG. 5 shows results of evaluation of variation in the breakdown voltageand the saturation voltage corresponding to variation in combination ofthe pitch of gate trenches 70 and distance (dx) from the side wall ofgate trench 70 to insulating layer 15.

As can be seen from FIG. 5, when the pitch of gate trenches 70 is 5.3μm, the saturation voltage increases in accordance with increase of dxto 1 μm. By increasing the pitch of gate trenches 70 fourfold, however,the saturation voltage can be improved to a value similar to thatattained with the pitch of 5.3 μm and dx of 0.2 μm.

Therefore, when the gate trench 70 and insulating layer 15 are to beformed in a non-self-alignment manner in the method of manufacturing theIGBT, it is necessary to increase dx in some cases. Even in these cases,if dx is substantially smaller than 1 μm, an intended performance of theIGBT can be ensured.

As described above, distance dx between insulating layer 15 and the wallsurface of gate trench 70 must be much shorter than the pitch of gatetrenches 70 in order to improve the carrier density. When dx is nearlyone-twentieth of the pitch, a sufficient effect can be achieved. Even ifdx cannot be reduced due to allowable processing accuracy, a sufficienteffect can be achieved with dx nearly equal to one tenth of the gatetrench pitch.

Based on the characteristics of the IGBT of this embodiment describedabove, a high-breakdown-voltage IGBT of the gate trench type wasprepared using one of combinations of parameters including the optimizedbreakdown voltage and the optimized saturation voltage. The saturationvoltage characteristics of this high-breakdown-voltage IGBT of the gatetrench type were compared with those of the IGBT having a conventionalstructure. Results are shown in FIG. 6 as waveform with circles. In theIGBT of this embodiment, gate trenches 70 are arranged with a pitch of5.3 μm, and each have a depth of 5 μm and a width of 1 μm. X is 0.7 μm,and dx is 0.2 μm. The insulating layer has thickness (Y') of 0.2 μm anddepth (Y) of 3.5 μm. Introduction of holes from p-collector layer 3 iscontrolled to set the saturation voltage to about 3 V with the collectorcurrent density of 100 A/cm².

As can be seen from FIG. 6, the IGBT of this embodiment canadvantageously reduce the saturation voltage at low current density, andthe on-state loss with a practical current density (lower than therating), which must be taken into consideration for application topractical circuits, can reduced.

The turn-off operation of the inductive load was evaluated with the IGBTdescribed above. The results are shown in FIG. 7. As compared with theevaluation of the conventional structure shown in FIG. 48, the turn-offloss is reduced to about 40% although nearly equal saturation voltagesare used.

In particular, such a problem of the conventional structure issubstantially overcome that delay in voltage rise occurs at an area ofthe collector voltage (Vce) higher than about 1200 V. The internaltemperature rise during a period from the on-state to completion of theturn-off state was calculated. From results of this calculation, It wasalso found that the temperature rise in the IGBT of the embodiment wassmaller by about 40% than that in the conventional IGBT.

A first practical example of a method of manufacturing the IGBTaccording to the above embodiment 1 will be described below withreference to FIGS. 9 to 20, which show sectional structures of the IGBTshown in FIG. 1 at various steps, respectively.

Referring first to FIG. 9, insulating layer 15 having a thickness oft_(ox) and made of an oxide film is formed on an n⁻ silicon substrate 1Ahaving a thickness from 400 to 630 μm and an impurity concentration from200 to 1000 Ω·cm. Insulating layer 15 is formed by wet or dry oxidationunder the condition from 820 to 1215° C. Film thickness t_(ox) ofinsulating layer 15 is preferably equal to or smaller that a quadrupleof that of gate insulating film 7 formed in gate trench 70.

There is also prepared an n⁻ silicon substrate 1B from 3 to 50 μm inthickness having the same impurity concentration as n⁻ silicon substrate1A.

Referring to FIG. 10, silicon substrate 1B is laminated over insulatinglayer 15 on n⁻ silicon substrate 1A to complete n⁻ silicon substrate 1.

Surfaces at the upper and lower sides of n⁻ silicon substrate 1 will bereferred to as first and second main surfaces, respectively.

Referring to FIG. 11, p-well 4 which is formed at a depth from 1.5 to4.0 μm and has a peak concentration of p-type impurity from 1×10¹⁵ to5×10¹⁸ cm⁻³ is formed at the first main surface of silicon substrate 1.n⁺ emitter regions 5 having a depth from 0.8 to 2.0 μm and a surfaceimpurity concentration from 1×10¹⁹ to 1×10²⁰ cm⁻³ are formed atpredetermined regions in the surface of p-well 4.

At and near the second main surface of n⁻ silicon substrate 1, n⁺ bufferlayer 2 and p⁺ collector electrode 3 are formed. n⁺ buffer layer 2 has adepth from 10 to 30 μm and a peak impurity concentration from 1×10¹⁴ to1×10¹⁸ cm⁻³. p⁺ collector layer 3 has a peak impurity concentrationhigher than that of n⁺ buffer layer 2.

Referring to FIG. 12, oxide film 26 having a predetermined pattern isformed on p-well 4. Using oxide film 26 as a mask, anisotropic dryetching is effected to open gate trench grooves 7a reaching insulatinglayer 15. Each gate trench groove 7a thus formed has a width (t_(w))from about 0.8 to about 3.0 μm and a depth from about 3.0 to about 15.0μm. The depth of gate trench groove 7a is a parameter depending onthickness (t_(ox)) of insulating layer 15.

Referring to FIG. 13, a deposition film (not shown) formed at gatetrench groove 7a is removed after formation of gate trench groove 7ashown in FIG. 12. In this step, each edge of the oxide film forminginsulating layer 15 is also removed laterally by a length dx.

Referring to FIG. 14, a silicon film 16 having a thickness of dx and thesame impurity concentration as n⁻ silicon substrate 1 is formed overinner surfaces of gate trench grooves 7a by an epitaxial growth method.In this step, as shown in FIG. 15, heat treatment during the epitaxialgrowth diffuses impurity in n⁺ emitter region 5 and p-well 4 intosilicon layer 16.

Silicon film 16 formed by the epitaxial growth may be replaced withpolycrystalline silicon which has a high resistance and is formed ofsame material as n⁻ silicon substrate 1.

Referring to FIG. 16, gate insulating films 7 are formed in gate trenchgrooves 7a, for example, by heat oxidation. It is preferable to ensuresuch a relationship that the film thickness of insulating layer 15 issubstantially smaller than a quadruple of that of gate insulating film7, as already described.

Before forming gate insulating film 7 but after forming gate trenchgroove 7a, isotropic plasma etching and sacrifice oxide may be formed sothat characteristics of the trench MOS and gate insulating film 7 can beimproved. More specifically, if an opening and a bottom of gate trenchgroove 7a had sharp edges or corners, local reduction in thickness ofgate oxide film 7 and concentration of electric field would occur. Incontrast to this, round opening and bottom of gate trench groove 7ashown in FIG. 16 can suppress concentration of the electric field.

Referring to FIG. 17, gate trench grooves 7a are filled with aconductive material 8a such as doped n-type polycrystalline silicon by aCVD method or the like. Thereafter, conductive material 8a and gateinsulating film 7 are patterned to expose n⁺ emitter regions 5 andp-wells 4 as shown in FIG. 18. In this manner, each trench gate 70formed of gate trench groove 7a, gate insulating film 7 and gateelectrode 8 is completed.

Referring to FIG. 19, a silicate glass (BPSG) film 19 and a CVD film 20,which contains boron and phosphorus having a good coating performance.Thereafter, silicate glass film 19 and CVD oxide film 20 are etched toform contact holes 20A each exposing expose n⁺ emitter region 5 andp-well 4.

Referring to FIG. 20, after forming contact holes 20A, emitter electrode10 electrically connected to n⁺ emitter regions 5 and p-wells 4 isformed on the whole surface above the first main surface of n⁻ siliconsubstrate 1.

Collector electrode 11 is formed on p-collector 3 over the second mainsurface of silicon substrate 1. In this manner, the IGBT according tothe embodiment 1 shown in FIG. 1 is completed.

A second specific example of the IGBT according to the embodiment 1 willbe described below with reference to FIGS. 21 to 29.

Referring to FIG. 21A, a pattern formed of insulating layers 15 eachhaving a width of t_(w) +2dx is formed on an n⁻ silicon substrate 1Ahaving an impurity concentration from 200 to 1000 Ω·cm. Using insulatinglayer 15, n⁻ silicon substrate 1A is patterned to form concavities 1C oft_(ox) in depth, as shown in FIG. 22A.

Referring to FIG. 21B, another insulating layer 15 made of, e.g., anoxide film and having a film thickness of t_(ox) is deposited on an n⁻silicon substrate 1B having the same impurity concentration as siliconsubstrate 1A. A resist film 22 of a pattern having an open width t_(w)+2dx is formed on insulating layer 15. Insulating layer 15 is patterned,and then resist film 22 is removed as shown in FIG. 22B.

Referring to FIG. 23, n⁻ silicon substrates 1A and 1B are washed afterremoving insulating layer 15 on n⁻ silicon substrate 1A. n⁻ siliconsubstrates 1A and 1B are laminated together as shown in FIG. 23, andheat treatment is effected on them at a temperature from 850 to 1100° C.in an atmosphere of O₂.

Referring to FIG. 24, the same step as that of the specific example 1shown in FIG. 11 is executed to form p-well 4, n⁺ emitter regions 5,n-buffer layer 2 and p-collector layer 3. n-buffer layer 2 andp-collector layer 3 may be formed in advance at n⁻ silicon substrate 1B.

Although p-collector layer 3 is formed on a whole area of the secondmain surface of n⁻ silicon substrate 1, n-type diffusion layer orlightly doped p-diffusion layer may be locally arranged, so that aperformance of the IGBT can be improved.

Referring to FIG. 25, a CVD oxide film 26 and having an appropriateopening pattern is formed on p-well 4. Using oxide film 26 as a mask,gate trench grooves 7a extending between insulating layers 15 areformed. In this example of the embodiment, gate trench groove 7a has awidth (t_(w)) from about 0.8 to about 3.0 μm and a depth from about 3.0to about 15.0 μm. Here, the depth of gate trench groove 7a is aparameter depending on the thickness of insulating layer 15. A distancebetween gate trench groove 7a and insulating layer 15 is defined as dx.

Referring to FIG. 26, gate insulating film 7 is formed on inner surfacesof gate trench grooves 7a. Similarly to the practical example 1 alreadydescribed, a sacrifice oxide film is formed and processing such asisotropic plasma etching and sacrificial oxidation or the like isexecuted before forming gate insulating film 7 but after forming gatetrench grooves 7a, so that the opening and bottom of each gate trenchgroove 7a are rounded, and irregular side wall of gate trench groove 7ais smoothed. Therefore, properties of the trench MOS and gate insulatingfilm 7 can be improved.

Referring to FIG. 27, a conductive material 8a made of, e.g., dopedn-type polycrystalline silicon is deposited in gate trench groove 7a.Thereafter, as shown in FIG. 28, conductive material 8a and gateinsulating film 7 are patterned into predetermined configurations, sothat gate electrode 8 is completed and thus trench gate 70 formed ofgate trench groove 7a, gate insulating film 7 and gate electrode 8 iscompleted. Thereafter, silicate glass film 19 covering only gate trench70 as well as CVD oxide film 20 are formed, and contact hole 20A isformed.

Referring to FIG. 29, emitter electrode 10 which is electricallyconnected to n⁺ emitter region 5 and p-well 4 is formed on the firstmain surface of n⁻ silicon substrate 1, and collector electrode 11 isformed on the surface of p-collector layer 3 formed over the second mainsurface of n⁻ silicon substrate 1. In these manners of the secondpractical example, the IGBT shown in FIG. 1 can be fabricated.

In the IGBT of the embodiment 1, insulating layer 15 functions as a kindof capacitor during the off state. Electrons are attracted onto theupper surface of insulating layer 15, so that strongly negative spacecharges are formed. An electric field, which is gradually increased bydonor ions coming from a lower side of n⁻ silicon substrate 1 and tendsto be applied to p-well 4, is intercepted by the above strongly negativespace charges, so that an electric field is not substantially presentbetween insulating layer 15 and p-well 4. Thereby, the potential at theupper surface of insulating layer 15 lowers to a potential nearly equalto the potential at and under emitter region 5 connected to p-well 4.

The potential at the lower surface of insulating layer 15 rises to anextent corresponding to the voltage drop in insulating layer 15, andthis potential rise is proportional to the thickness of insulating layer15. Therefore, this potential rise can be suppressed to allow only aslight rise by reducing the thickness of insulating layer 15. Meanwhile,a potential not higher than the emitter potential is set in gate trench70, and the electric field is enhanced at the corner of the bottom ofgate trench 70. However, if the end of insulating layer 15 is close tothe bottom corner of the trench, a potential difference between them issmall, because a lower potential is carried under insulating layer 15.Thereby, the electric field is reduced, so that the breakdown voltagecan be improved.

As described above, in order to reduce a difference between thepotential under insulating layer 15 and the potential at gate trench 70,thinner insulating layer 15 is advantageous in view of the breakdownvoltage. According to the evaluation results, it can be considered thatthe optimum thickness of insulating layer 15 is substantially smallerthan a quadruple of the thickness of gate insulating film 7 of gatetrench 70.

Optimum distance dx between the wall surface of gate trench 70 andinsulating layer 15 is substantially equal to the thickness ofinsulating layer 15. If distance dx is excessively small, the breakdownvoltage decreases. It is desirable that the depth of insulating layer 15is substantially equal to the depth of gate trench 70 from the viewpointof the breakdown voltage.

During the on state of the IGBT, the insulating layer 15 functions toprevent attraction of holes into p-well 4. Since gate trench 70 isbiased strongly positively, electrons are attracted onto the wall ofgate trench 70, and holes are repelled. Therefore, holes cannot easilymove through a gap (dx) between the wall of gate trench 70 andinsulating layer 15, and thus cannot easily arrive at p-well 4.

For the above reason, the hole loss at the emitter side decreases, andan efficiency of introduction of electrons from the trench channelincreases, so that a large amount of electrons and holes are suppliedinto n⁻ silicon substrate 1, which improves the electric conductance andreduces the saturation voltage. Therefore, in order to decrease thesaturation voltage in this manner, it is necessary to narrow a gapbetween the wall of gate trench 70 and insulating layer 15, and for thispurpose, insulating layer 15 must be shallower than the gate trench 70.

As the pitch of gate trench 70 increases, a ratio of dx to the pitchdecreases, and holes are suppressed from reaching p-well 4 to a furtherextent, so that the carrier density increases. However, if the pitchwere excessively large, holes would disappear due to recombinationbetween them, resulting in reduction of the carrier density.

As described above, when the density of carriers in n⁻ silicon substrate1 increases at the emitter side during the on state of the IGBT, theconductivity of n⁻ silicon substrate 1 increases, and the saturationvoltage lowers. In this case, when the supply of holes from p-collector3 is reduced to restore the saturation voltage, such a distribution isestablished that the carrier density at the emitter electrode side ishigher than that at the collector electrode side as shown in FIG. 8.

Operation during the turn-off of the IGBT will now be discussed below.In general, when the channel can supply sufficient electrons no longerdue to lowering of the gate voltage, and thereby the collector voltagestarts to rise, excessive holes stored in n⁻ silicon substrate 1 areattracted toward gate trench 70 carrying a lower voltage. As a result,holes move along the wall of gate trench 70 to p-well 4.

Accordingly, a large current would not flow through a portion underp-well 4 surrounded by gate trenches 70 during the turn-off state evenif insulating layer 15 were not present at this portion, so thatexistence of insulating layer 15 at this portion does not cause aparticular problem.

When the IGBT which has the carrier distribution shown in FIG. 8 duringthe on state is turned off, many holes which existed at the emitterelectrode side are discharged from the emitter electrode side, so thatthe depletion layer formed by discharging holes extends only slowly atan initial stage during the turn-off, and the collector voltage startsto rise relatively slowly.

When the collector voltage rises to a certain extent and the depletionlayer extends, the end of the depletion layer reaches a regioncontaining only a small amount of carriers which were initiallyaccumulated, so that extension of the depletion layer owing to dischargeof holes occurs rapidly.

Similarly to the IGBT of the conventional structure, the electric fieldin the depletion layer is enhanced by modulation with a differencebetween charge densities of electrons and holes forming the current, andimpact-generated carriers temporarily supply an electron current todelay the turn-off. In this case, however, the difference in densitybetween holes and electrons is slightly small, so that the above delayoccurs only to a small extent.

As a result, the collector voltage continues rapid rising until the endof turn-off. As shown in FIG. 7, therefore, the turn-off loss isreduced, and it is possible to suppress rise in temperature inside theIGBT due to the turn-off loss.

In FIG. 7, the gate trench pitch is four times as large as the referencepitch, so that the gate capacity is reduced to a quarter correspondinglyto reduction in gate trench number per area. Although the gate driveresistance which is used for evaluating the turn-off operation isincreased fourfold as compared with the reference IGBT, the example inFIG. 7 can perform turn-off slightly but more rapidly, so that it canachieve an effect of reducing the gate capacity.

Further, this embodiment can employ the gate trench grooves with anincreased pitch, and thereby can improve the performance.

In this embodiment, the thin insulating layer is located near the bottomcorner of the gate trench to keep a low potential above the insulatinglayer, which is an important point for improving the performance, as canbe understood from the above description. This condition can be achieveby structures other than the insulating layer buried in a planarfashion, and embodiments 2 and 3 which will be described below can beemployed as practical application.

(Embodiment 2)

A semiconductor device with a high breakdown voltage and a method ofmanufacturing the same according to the embodiment 2 of the inventionwill be described below.

Referring to FIG. 30, a sectional structure of a high-breakdown-voltageIGBT of a gate trench type will be described below as an example of asemiconductor device with a high breakdown voltage of the embodiment 2.Portions and parts bearing the same reference numbers as those of theembodiment 1 have the same functions.

In addition to the structure of IGBT of the embodiment 1, the structureof IGBT of the embodiment 2 includes an emitter trench 80 between gatetrenches 70.

Emitter trench 80 has an emitter trench groove 80a having the same depthas gate trench groove 7a of gate trench 70, an emitter insulating film80 covering an inner surface of emitter trench groove 80a, and anemitter trench electrode 80c filling emitter trench groove 80a and madeof doped polycrystalline silicon. Emitter trench electrode 80c iselectrically connected to emitter electrode 10.

One prepared IGBTs which employed the above structure, and each haddistance dx of 0.2 μm between gate trench 70 and emitter trench 80. Inthe IGBTs, the pitch of gate trenches 70 were set to a reference valueof 5.2 μm and 2.4 μm, respectively. The saturation voltages andbreakdown voltages of these IGBTs were compared with those of theconventional structure. Results are shown in FIG. 31. (In the graph,"Dummy" corresponds to the values for the present structure. Thestandard TIGBT having a pitch of 5.3 μm corresponds to the structurehaving dx of 4.3 μm.)

As compared with the standard IGBT, each of the structures having dx of0.2 μm had a reduced saturation voltage and an increased quantity ofcarriers introduced into silicon substrate 1. It can also be seen thatthe breakdown voltage slightly increased and therefore the IGBT had animproved performance.

A method of manufacturing the IGBT of the embodiment 2 thus constructedwill be described below with reference to FIGS. 32 to 40. FIGS. 32 to 40are cross sections corresponding to that of FIG. 30, and showmanufacturing steps, respectively.

Referring to FIG. 32, a step is performed to prepare n⁻ siliconsubstrate 1 having an impurity concentration from 200 to 1000 Ω·cm.

Referring to FIG. 33, processing similar to that of the embodiment 1 isperformed to form p-well 4 and n⁺ emitter regions 5 at and near thefirst main surface of n⁻ silicon substrate 1. p-well 4 has a depth from1.5 to 4.0 μm, and an impurity peak concentration from 1×10¹⁵ to 5×10¹⁸cm⁻³. n⁺ emitter regions 5 have a depth from 0.8 to 2.0 μm and a surfaceimpurity concentration from 1×10¹⁹ to 1×10²⁰ cm⁻³.

n-buffer layer 2, which has a depth from 10 to 30 μm and a peakconcentration from 1×10¹⁴ to 1×10¹⁸ cm⁻³, and p-collector layer 3, whichhas a depth from 3 to 10 μm, and an impurity peak concentration higherthan that of n-buffer layer 2, are formed at and near the second mainsurface of n⁻ silicon substrate 1.

Referring to FIG. 34, processing is performed to form gate trenchgrooves 7a extending through n⁺ emitter regions 5 and emitter trenchgroove 80a located between n⁺ emitter regions 5. Referring to FIG. 35,processing such as isotropic plasma etching and formation of a sacrificeoxidation film is performed after forming gate trench grooves 7a andemitter trench groove 80a, so that the openings and bottoms of gatetrench grooves 7a as well as the opening and bottom of emitter trenchgroove 80a are rounded, and irregular surfaces of the side walls of gatetrench grooves 7a and emitter trench groove 80a are smoothed.Consequently, it is possible to improve characteristics of theinsulating films formed at the inner surfaces of gate trench grooves 7aand emitter trench groove 80a.

Referring to FIG. 36, an insulating film 7b, which is made of SiO₂ orthe like and will form gate insulating films 7 and emitter insulatingfilm 80b, is formed in gate trench grooves 7a and emitter trench groove80a.

Referring to FIG. 37, a step is performed to fill gate trench grooves 7aand emitter trench groove 80a with a conductive material 8b made ofdoped n-type polycrystalline silicon.

Referring to FIG. 38, insulating film 7b and conductive material 8b arepatterned into predetermined configurations to form gate trenches 70,each of which is formed of gate trench groove 7a, gate insulating film 7and gate electrode 8, and emitter trench 80, which is formed of emittertrench groove 80a, emitter insulating film 80b and emitter trenchelectrode 80c.

Referring next to FIG. 39, a silicate glass mark 19 and a CVD oxide film20 are formed, and contact holes 20A and 50 are opened. FIG. 41 is a topview showing the pattern of the structure at this time. Contact hole 20is formed in the region surrounded by lines A-A'" and B-B'". Inaddition, n type doped polycrystalline silicon 8b is etched betweenlines A-A'" and B-B'", n type doped polycrystalline silicon 80c and 8being electrically separated from one another.

As shown in FIG. 40, processing such as sputtering is performed to formemitter electrode 10, which is located above the first main surface ofn⁻ silicon substrate 1 and is electrically connected to n⁺ emitterregion 5, p-well 4 and emitter trench electrode 80c. Processing such assputtering is also performed to form collector electrode 11 at thesurface of p-collector layer 3 over the second main surface of siliconsubstrate 1. In this manner, the IGBT of the embodiment 2 shown in FIG.30 is completed.

If the structure of IGBT of the embodiment 2 is miniaturized to a higherextent, dx decreases and, in the same sectional plane, p-well 4 and n⁺emitter region 5 cannot contact with emitter electrode 10 in some cases,as can be seen from the structural section in FIG. 30.

In these cases, p-wells 4 and n⁺ emitter regions 5 are alternatelyarranged as shown in a plan of FIG. 41, whereby the IGBT can actuallyhave a miniaturized structure. The structure shown in FIG. 41 is takenalong line 41--41 in FIG. 40.

As described above, the IGBT of the embodiment 2 can achieve theoperation and effect similar to those of the IGBT of the embodiment 1,and further can ensure a high carrier introduction performance and anintended breakdown voltage merely by reducing dx without requiring allthe gate trenches to be set to the gate potential.

It may seem that even a conventional structure can achieve an effectsimilar to that of the structure of the embodiment 2 by reducing thepitch and dx. However, according to the above embodiment, employment ofthe emitter trench reduces an area of gate trenches per unit area, sothat the gate capacity can be reduced significantly. In particular,reduction in capacity (feedback capacity) between the gate and thecollector allows rapid switching, so that the switching loss can bereduced. This effect can be achieved not only by this embodiment butalso but the embodiment 1 already described and the embodiment 3 to bedescribed later.

In semiconductor devices with high breakdown voltages which are employedfor handling a high power, reduction in the gate capacity has beenstrongly demanded for simplifying the system. In view of this, theeffect of this embodiment is practically very advantageous. The emittertrench structure allows formation of different kinds of trenches, i.e.,gate trench and emitter trench only by changing a manner of connectionto electrodes buried in the trenches. Therefore, the structure of thisembodiment can be manufactured more easily than the structure of theembodiment 1.

(Embodiment 3)

A semiconductor device with a high breakdown voltage according to theembodiment 3 of the invention and a method of manufacturing the samewill be described below.

Referring first to FIG. 42, a sectional structure of an IGBT will bedescribed below as an example of the semiconductor device with the highbreakdown voltage according to the embodiment 3. In FIG. 42, parts andportions having the same functions as those of the embodiment 2 bear thesame reference numbers. In FIG. 42, p-wells 4 arranged between emittertrenches 80 do not affect the operation of IGBT, and therefore are notessential, so that n⁻ silicon substrate 1 without these p-wells 4 may beemployed.

In contrast to the IGBT of the embodiment 2 in which one emitter trench80 is arranged between adjacent two gate trenches 70, the IGBT of theembodiment 3 has such a structure that a plurality of emitter trenches80 adjacent to each other are arranged between two gate trenches 70located at predetermined positions.

In this case, a ratio of distance dx between gate trench 70 and emittertrench 80 to the pitch of gate trenches 70 can be reduced to an intendedvalue, even if gate trench 70 and emitter trench 80 have the sameconfigurations. Therefore, this structure can be manufactured moreeasily than the structure of the embodiment 2.

For example, when one intends to complete a structure in which thetrench width and dx are 1 μm, and a ratio of dx to the pitch of gatetrenches 70 is 1:20, this structure can be achieved by disposing eachgate trench 70 at every ten positions for trenches. By employing thisstructure, the gate capacity can be reduced approximately to a quarterof that of the standard IGBT of the gate trench type, and the gatecapacity can be reduced to one-tenth that of the structure shown in FIG.50 having the same pitch.

Description will be given on a method of manufacturing the IGBT of theembodiment 3 thus constructed with reference to FIGS. 43 to 46. FIGS. 43to 46 show sectional structures in different manufacturing steps,respectively.

Referring to FIG. 43, steps similar to those of the embodiment 2 shownin FIGS. 32 to 38 are performed to form gate trenches 70 and emittertrenches 80. Referring to FIG. 44, an oxide film 18 is then formed tocover only the surfaces of gate electrodes 8 of gate trenches 70.

Referring to FIG. 45, silicate glass film 19 and CVD oxide film 20covering gate trenches 70 are formed, and silicate glass films 10Acovering only portions of the p-wells exposed between the emittertrenches are formed.

Referring to FIG. 46, emitter electrode 10 is formed entirely over thefirst main surface of n⁻ silicon substrate 1, and collector electrode 11is formed over p-collector layer 3 formed at the second main surface ofn⁻ silicon substrate 1. Through these steps, the IGBT of the embodiment3 shown in FIG. 42 is completed.

The IGBT of the embodiment 3 can achieve an effect similar to that bythe embodiments 1 and 2. Further, the structure of the embodiment 3 canemploy the planar structure of the embodiment 2 shown in FIG. 41, ifp-wells 4 and emitter regions 5 cannot be arranged on the same sectiondue to miniaturization of the devices.

Here, not only IGBT having the cross sectional structure shown in FIG.42 but also IGBTs having cross sectional structures shown in FIGS. 47and 48 can also be adopted. In the IGBT shown in FIG. 47, two gatetrenches are provided continuously, with n⁺ emitter region 5 provided ata portion of p well 4 between gate trenches 70 and in contact with gatetrench 70. Between the gate trenches 70, one or more emitter trench 80and p well 4 are provided in repetition. Owing to this structure,exposure rate of p well 4 is reduced, thus improving the carriersupplying capability of the emitter trench 80. In addition, an effectsimilar to what is obtained in the structure of FIG. 47 can also beobtained even when a structure without p wells 4 at the opposing endportions of emitter trench 80 is adopted, as shown in FIG. 48.

It is clearly understood that all the embodiments described above are byway of illustration and example only and are not to be taken by way oflimitation. Although sectional trench structures have been discussed inconnection with the embodiments 1 to 3, the invention can be applied notonly to the structure including straight gate trench grooves but also toother gate trench grooves, such as ring-shaped grooves or cell-shapedgrooves.

Although the n-channel IGBTs using n⁻ silicon substrates have beendiscussed, the invention can be applied to IGBTs having an oppositepolarity, i.e., p-channel channel IGBTs. The invention can be employedin thyristor-type elements with insulating gates for increasing carriersintroduced into substrates.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A semiconductor device comprising an insulatedgate bipolar transistor of a gate trench type having a high breakdownvoltage comprising:a semiconductor substrate of a first conductivitytype having first and second main surfaces; gate trenches, each having afirst groove formed at a predetermined region of said first main surfaceand extending from said first main surface into said semiconductorsubstrate to a depthwise direction, a gate insulating film covering aninner surface of said first groove, and an electrode filling said firstgroove and made of an electrical conductor; an impurity region of thefirst conductivity type formed near said first main surface and locatedin the vicinity of said gate trench; a first main electrode layercovering said first main surface, opposed to said gate trench with aninsulating film therebetween and electrically connected to said impurityregions and said semiconductor substrate; a second impurity layer of thesecond conductivity type formed at said second main surface; and asecond main electrode layer formed at a surface of said second impuritylayer, wherein each gate trench is spaced from another gate trench witha predetermined pitch, and a plurality of emitter trenches, not gatetrenches, arranged between two gate trenches, each emitter trench havinga second groove extending from said first main surface into saidsemiconductor substrate in a depthwise direction, an insulating filmcovering an inner surface of said second groove and second electrodefilling said second groove are arranged at positions between said gatetrenches, wherein a surface of the impurity region of the firstconductivity type formed near the main surface exposed between emittertrenches is covered with an interlayer insulating film.
 2. Thesemiconductor device with a high breakdown voltage according to claim 1,whereinsaid semiconductor substrate further includes a first impuritylayer of the second conductivity type extending from said first mainsurface into said semiconductor substrate in a depthwise direction. 3.The semiconductor device with a high breakdown voltage according toclaim 1, whereinsaid impurity region of the first conductivity type isformed as a pair, sandwiching said gate trench.
 4. A semiconductordevice comprising an insulated gate bipolar transistor of a gate trenchtype having a high breakdown voltage comprising:a semiconductorsubstrate of a first conductivity type having first and second mainsurfaces; gate trenches, each having a first groove formed at apredetermined region of said first main surface and extending from saidfirst main surface into said semiconductor substrate to a depthwisedirection, a gate insulating film covering an inner surface of saidfirst groove, and an electrode filling said first groove and made of anelectrical conductor; an impurity region of the first conductivity typeformed near said first main surface and located in the vicinity of saidgate trench; a first main electrode layer covering said first; mainsurface, opposed to said gate trench with an insulating filmtherebetween and electrically connected to said impurity regions andsaid semiconductor substrate; a second impurity layer of the secondconductivity type formed at said second main surface; and a second mainelectrode layer formed at a surface of said second impurity layer,wherein each gate trench is spaced from another gate trench with apredetermined pitch, and a plurality of emitter trenches, not gatetrenches, arranged between two gate trenches, each emitter trench havinga second groove extending from said first main surface into saidsemiconductor substrate in a depthwise direction, an insulating filmcovering an inner surface of said second groove and second electrodefilling said second groove are arranged at positions between said gatetrenches, wherein a distance between a side wall of said emitter trenchand a side wall of said gate trench is equal to or smaller thanone-tenth of the pitch of said gate trenches.